Many types of pigment materials, or fillers, are used in the paper industry to opacify the paper. The most common filler is clay, which has the advantage of being low in cost but the disadvantage of having relatively poor opacifying power. To manufacture papers of high opacity, the industry uses large amounts of anatase TiO.sub.2 which has the advantage of having very high opacifying power but the disadvantage of being relatively very expensive.
So long as paper was manufactured under acid conditions because of the need to use alum to obtain sizing with rosin, the paper industry could not use CaCO.sub.3 as a filler. Fine particle size precipitated CaCO.sub.3 has excellent characteristics as a potential filler for paper. It has about half the opacifying power of TiO.sub.2 but at a small fraction of the cost of TiO.sub.2. Indeed, with the advent recently of neutral and alkaline sizing materials, those paper mills which have converted to neutral or alkaline paper making have found that by using precipitated CaCO.sub.3, the use of TiO.sub.2 can be greatly curtailed with resulting significant cost savings.
To accurately describe the present invention, it is necessary to have a quantitative means to express and compare the opacifying power of filler as used in paper. This can be done accurately and precisely by using a measurement called the scattering coefficient, s, derived from the theory of light scattering developed by Kubelka and Munk and published in 1931, Z. Tech. Physik 12:539 (1931). Application of the Kubelka-Munk Theory to papers did not come until much after 1931, but is now the common means used by the paper industry to evaluate the opacifying power of fillers. There are a number of excellent discussions of paper applications such as that by W. J. Hillend, Tappi, 49:41A (July 1966) and by G. A. Hemstock, Tappi, 45:158A (February 1962).
Table 1 below gives the scattering coefficients in paper, s, for clay, precipitated CaCO.sub.3, and the two forms of TiO.sub.2. The higher the value of s, the greater the opacifying power of the filler. Examination of the scattering coefficients in Table 1 will show that for clay, precipitated CaCO.sub.3 and anatase TiO.sub.2, these are very close to being in the ratio 1:2:4. It is well recognized that rutile TiO.sub.2 has even more opacifying power than anatase, but it is also even more expensive and is used but little in paper.
Table 1 also gives the scattering coefficients in paper for two ground natural limestones, ground limestone A whose average particle size is 3.8u and ground limestone B whose average particle size is 2.0u. As the coefficients show, these have about the same opacifying power as clay. They can be used in place of clay in alkaline papers but they do not give the reduction in TiO.sub.2 requirements experienced with precipitated CaCO.sub.3. Still there is must interest in the paper industry in using ground natural limestones because these are in wide production while there are only a few producers of precipitated CaCO.sub.3, and the natural products are cheaper than the precipitated.
TABLE 1 ______________________________________ Scattering Coefficients in Paper s Filler cm.sup.2 /g Identification ______________________________________ filler clay 1100 Klondike, water washed Engelhard Minerals & Chemicals Corporation anatase TiO.sub.2 4700 Titanox AWD 1010 NL Industries Inc. or Anatase A-410 New Jersey Zinc Company rutile TiO.sub.2 5500 Titanox RA 42 NL Industries Inc. or Rayox R-77 R. T. Vanderbilt Co. ground limestone A 1200 3.8.mu. average particle size ground limestone B 1500 2.0.mu. average particle size precipitated CaCO.sub.3 2300 Purecal-O, BASF, Wyandotte or Albaglos, Pfizer, Inc. 1.0.mu. average particle size ______________________________________